Add hotkeys

Typing `space` should open the first card shown. `r` should reset filters. `1` to `6` should trigger the 6 example filters.
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The cool thing would be to have `?` bring up a modal showing the shortcuts, as works with Gmail and Google Calendar. Of course, you're right about `space`; `return` would be a better choice. Interestingly, `esc` already works to dismiss the modal.